How load average is calculated? When is it right to say that load on the server is high?

Actually there are two concepts related to system load on Linux

• System load/CPU Load – is a measurement of CPU over or under-utilization in a Linux system; the number of processes which are being executed by the CPU or in waiting state.

• Load average – is the average system load calculated over a given period of time of 1, 5 and 15 minutes.

In Linux, the load-average is technically believed to be a running average of processes in it’s (kernel) execution queue tagged as running or uninterruptible.

Note that:

• All if not most systems powered by Linux or other Unix-like systems will possibly show the load average values somewhere for a user.

• A downright idle Linux system may have a load average of zero, excluding the idle process.

• Nearly all Unix-like systems count only processes in the running or waiting states. But this is not the case with Linux, it includes processes in uninterruptible sleep states; those waiting for other system resources like disk I/O etc.

We can’t possibly explain system load or system performance without shedding light on the impact of the number of CPU cores on performance.

Multi-processor Vs Multi-core

• Multi-processor – is where two or more physical CPU’s are integrated into a single computer system.

• Multi-core processor – is a single physical CPU which has at least two or more separate cores (or what we can also refer to as processing units) that work in parallel. Meaning a dual-core has 2 two processing units, a quad-core has 4 processing units and so on.

Furthermore, there is also a processor technology which was first introduced by Intel to improve parallel computing, referred to as hyper threading.

Under hyper threading, a single physical CPU core appears as two logical CPUs core to an operating system (but in reality, there is one physical hardware component).

Note that a single CPU core can only carry out one task at a time, thus technologies such as multiple CPUs/processors, multi-core CPUs and hyper-threading were brought to life.

With more than one CPU, several programs can be executed simultaneously. Present-day Intel CPUs use a combination of both multiple cores and hyper-threading technology.

On a single core system this would mean:

• The CPU was fully (100%) utilized on average; 1 processes was running on the CPU (1.00) over the last 1 minute.

• The CPU was idle by 60% on average; no processes were waiting for CPU time (0.40) over the last 5 minutes.

• The CPU was overloaded by 235% on average; 2.35 processes were waiting for CPU time (3.35) over the last 15 minutes.

On a dual-core system this would mean:

• The one CPU was 100% idle on average, one CPU was being used; no processes were waiting for CPU time(1.00) over the last 1 minute.

• The CPUs were idle by 160% on average; no processes were waiting for CPU time. (0.40) over the last 5 minutes.

• The CPUs were overloaded by 135% on average; 1.35 processes were waiting for CPU time. (3.35) over the last 15 minutes.

Thanks ;)